Thermosetting polyimides are commercially available as uncured resins, stock shapes, thin sheets, laminates, and machines parts. Thermoplastic polyimides are very often called pseudothermoplastic. There are two general types of polyimides. One type, so-called linear polyimides, is made by combining imides into long chains. Aromatic heterocyclic polyimides are the other usual kind, where R′ and R″ are two carbon atoms of an aromatic ring. Examples of polyimide films include Apical, Kapton, UPILEX, VTEC PI, Norton TH and Kaptrex. Polyimides have been in mass production since 1955. Typical monomers include pyromellitic dianhydride and 4,4′-oxydianiline.
Lightweight, low density structures are desired for acoustic and thermal insulation for aerospace structures, habitats, and astronaut equipment and aeronautic applications. Aerogel is a manufactured material with the lowest bulk density of any known porous solid. It is derived from a gel in which the liquid component of the gel has been replaced with a gas. The result is an extremely low-density solid with several properties, most notably its effectiveness as a thermal insulator and its extremely low density. It is nicknamed frozen smoke, solid smoke, or blue smoke due to its translucent nature and the way light scatters in the material; however, it feels like expanded polystyrene to the touch. Aerogels are produced by extracting the liquid component of a gel through supercritical drying. This allows the liquid to be slowly drawn off without causing the solid matrix in the gel to collapse from capillary action, as would happen with conventional evaporation. The first aerogels were produced from silica gels.
Plain silica aerogels are brittle. Reinforcing the aerogel structure with polymer provides improvements in strength while maintaining low density and pore structure. Degradation of polymers used in cross-linking tends to limit use temperatures to below 150° C.
Polyimide aerogels can be fabricated from linear polyimides by allowing a low concentration polyimide/polyamic acid solution to gel, followed by heating to complete imidization and subsequent supercritical fluid extraction, as taught for example by Wendall, R., et al., WO/2004/009673, and Chidambareswarapattar, C., et. al., J. Mater. Chem. 2010, 20, 9666-9678. Polyimide aerogels prepared in this way from, for example, oxydianiline and pyrolimellitic dianhydride, have high surface areas, low density, low thermal conductivity, and good ductility. However, the gels shrink substantially, up to 40%, during supercritical fluid extraction.
Polyimide aerogels can also be synthesized by reaction of dianhydrides with di-isocyanates instead of diamines, as also reported by Chidambareswarapattar, C., et. al., J. Mater. Chem. 2010, 20, 9666-9678. This approach resulted in less shrinkage if gels were allowed to cure at room temperature, but results of thermogravimetric analyses of these aerogels revealed that imidization had not gone to completion.
Polyimide aerogels can also be synthesized by cross-linking anhydride end-capped polyamic acid oligomers via aromatic triamines, followed by thermal imidization, as taught for example by Kawagishi, K., et al., Macromol. Rapid Commun. 2007, 28, 96-100, and Meador, M. A. B., et al., Polym. Prepr. 2010, 51, 265-266. Unfortunately, the thermal imidization caused the gels to re-dissolve to some extent, suggesting hydrolysis of amic acid and disruption of the integrity of the polyimide aerogel network.
Polyimide aerogels also can be synthesized by cross-linking anhydride end-capped polyamic acid oligomers via aromatic triamines, followed by chemical imidization, as taught for example by Meador, et al. U.S. Pat. No. 9,109,088, or by cross-linking amine end-capped polyamic acid oligomers via triacid chlorides or polymaleic anhydrides, as taught by Meador, et al., ACS Appl. Mater. Interfaces 2015, 7, 1240-1249, and Guo, et al., RSC Adv. 2016, 6, 26055-26065. The properties of these polyimide aerogels are mainly dominated by the backbone chemistries of the oligomers, rather than the cross-linkers. For example, use of 3,3′,4,4′-biphenyltetracarboxylic dianhydride (“BPDA”) in combination with 2,2′-dimethylbenzidine (“DMBZ”) in the oligomer backbone provides aerogels with a higher modulus at lower density due to the stiffness of the backbone, while 4,4′-oxydianiline (“ODA”) used with BPDA in the backbone provides a lower modulus material that results in more flexible thin films. A combination of 50 mol % DMBZ and 50 mol % ODA used as diamine in a backbone with BPDA provides some moisture resistance and affords enough flexibility in the backbone to make foldable, thin films.
Moisture resistance is needed in the polyimide aerogels because the porous structures typically do not remain intact if wetted and re-dried, limiting technical applications to ones that will not result in wetting and re-drying. Thus, improvements in moisture resistance would be desirable. Also, although thin films made from these polyimide aerogels can be flexible, monolithic objects made from these polyimide aerogels having thickness of about 2 to 3 mm or greater are stiff, not flexible, and thus are not suitable for conformal applications. Thus, improvements in flexibility would be desirable too.
Accordingly, a need exists for improved porous cross-linked polyimide networks and methods of making such networks. A need also exists for porous cross-linked polyimide aerogels and thin films comprising porous cross-linked polyimide aerogels.